Giant clam description

This enormous shellfish is the largest species of bivalvemollusc in the fossil record, and the heaviest of all the living molluscs(4). Like all bivalvemolluscs, the shell of the giant clam (Tridacna gigas) consists of two valves, although in the larger giant clams these cannot close completely (6). The shell is extremely thick and lacks bony plates; when viewed from above, each valve has four to five inward facing triangular projections (6). The mantle of the giant clam is visible between the two shells, and is a golden brown, yellow or green, although there may be such an abundance of small blue-green circles that the overwhelming impression is of a beautiful iridescent colour (6)(7). A number of pale or clear spots on the mantle, known as 'windows', allow sunlight to filter in through the mantle(6). The mantle of the giant clam is completely fused with the exception of two holes (or 'siphons'). The gills are visible through the inhalant siphon, while the exhalent siphon is tube-like and is capable of expelling a large volume of water during spawning, or if the giant clam’s shells close suddenly (5)(7).

Related species

Giant clam biology

Adult giant clams are completely sessile, unable to move from their position on the coral reef. They reproduce by expelling sperm and eggs into the sea (6), where fertilization occurs. The fertilised eggs quickly enter a swimming stage (where they are known as trochophores), before entering a planktonic stage (7). During this stage, the larvae, (known as 'veligers'), inhabit the open ocean for one week, before settling in the substrate. If a clam is disturbed it will close its shell valves (6).

Giant clams have an inhalant siphon, which they use to draw in seawater that is then filtered for planktonic food (6). The majority of the clam's nutrients, however, are obtained by a mutually beneficial relationship with minute algae known as zooxanthellae (6). These plant-like algae exist in delicate tubules which are extensions of the stomach (8). The algae gain protection from predation by being associated with such a large organism, while the clam obtains the carbon by-products of photosynthesis(9). Giant clams also provide protection for a species of pea crab (Xanthasia murigera); a single pair will often be found living within the cavity of the clam (5).

Giant clam status

Giant clam threats

Giant clams have been extensively harvested for their meat and to supply the aquarium trade with such exotic specimens (6). Unable to sustain this exploitation, populations are now showing signs of decline; Tridacna gigas have not been seen in Fiji for over 50 years, primarily as a result of past over-collection for food (2).

Giant clam conservation

These clams are listed on Appendix II of the Convention on International Trade in Endangered Species (CITES), which requires a permit to be granted before giant clams can be exported (3). There has been considerable success with farming (10), which may help to alleviate the pressure on wild populations in the long-term. Farmed clams may also be used in restocking programmes, where numbers have become severely restricted in the wild. Giant clams were reintroduced to Tongan waters in 1990 (2), from quarantined-reared stocks cultured in Australia under the Australian Centre for International Agricultural Research and James Cook University giant clam project (11). These enormous molluscs have inspired awe for centuries and effective protection measures are vital if they are to be adequately conserved for future generations.

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Glossary

Simple plants that lack roots, stems and leaves but contain the green pigment chlorophyll. Most occur in marine and freshwater habitats.

Bivalve

In this group of aquatic molluscs the soft parts are encased in a shell consisting of two parts known as valves.

Larvae

Stage in an animal’s lifecycle after it hatches from the egg. Larvae are typically very different in appearance to adults; they are able to feed and move around but usually are unable to reproduce.

Mantle

In molluscs, a fold of skin that encloses a space known as the mantle cavity, which contains the gills. The mantle is responsible for the secretion of the shell.

Molluscs

A diverse group of invertebrates, mainly marine, that have one or all of the following; a horny, toothed ribbon in the mouth (the radula), a shell covering the upper surface of the body, and a mantle or mantle cavity with a type of gill. Includes snails, slugs, shellfish, octopuses and squid.

Photosynthesis

Metabolic process characteristic of plants in which carbon dioxide is broken down, using energy from sunlight absorbed by the green pigment chlorophyll. Organic compounds are made and oxygen is given off as a by-product.

Planktonic

Aquatic organisms that drift with water movements; may be either phytoplankton (plants), or zooplankton (animals).

Siphon

In molluscs, a tube-like structure through which water passes into or out of the mantle cavity.

Spawning

The production or depositing of large quantities of eggs in water.

Trochophores

A type of marine larva with several bands of cilia (hair-like projections). By moving the cilia, the trochophore can control the direction of its movement.

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